The identification and utilization of specific parasite antigens to study host-parasite interactions continues to be a major focus of this laboratory. The rate of clearance of two specific schistosome excretory-secretory antigens depended upon the nature and composition of the antigen, intensity of infection, the specific antibody response of the host, and size or charge/mass ratio of the specific immune complexes. Secretory membrane components of G. lamblia were studied following radiolabelling of the surface. The major secretory product is a polydisperse hydrophobic material destroyed by proteases and Na periodate. The major secretory product(s) of the two human strains studied differed antigenically and one strain lacked a peptide labelled in the other. In crossed immunoelectrophoresis, the major secretory material was identified as a slightly anodic material present in 3 of the 4 strains tested.